Semiconductor apparatus and method for fabricating the same

ABSTRACT

First, a substrate, on which a plurality of semiconductor devices is formed, is provided. Next, a first etching treatment is carried out to the substrate with a first etching gas comprising CF 4  to form a base trench having a rounded-off upper edge or tapered upper edge. A second etching treatment is carried out to the substrate to form a trench region at the base trench so that the trench region has a rounded-off upper edge. And then, an insulating layer is formed on the substrate to fill up the trench region therewith.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority of Application No. 2000-161928, filed May 31, 2000 in Japan, the subject matter of which is incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a semiconductor apparatus, and more particularly to a method for fabricating a semiconductor apparatus having a trench region for isolating semiconductor devices in the apparatus.

BACKGROUND OF THE INVENTION

For isolating semiconductor devices in a semiconductor apparatus, trench isolation technique has been used. According to a conventional method, an SiN layer is formed on a semiconductor substrate; then trench regions are formed by etching using the SiN layer as a mask. After that, an oxide layer is provided in the trench regions by a CVD process; and then, the oxide layer is removed by a CMP process.

According to such a conventional method, sharp corners of the trench regions may be exposed when the SiN layer is removed. As a result, characteristics of semiconductor devices may be changed, and electric field is concentrated at the exposed corners of the trench regions.

OBJECTS OF THE INVENTION

Accordingly, an object of the present invention is to provide a semiconductor apparatus in which a trench region is fabricated to have an upper edge or upper corner which is not exposed.

Another object of the present invention is to provide a semiconductor apparatus in which a trench region is fabricated not to have sharp upper edge or upper corner.

Additional objects, advantages and novel features of the present invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, a semiconductor apparatus includes a substrate on which a plurality of semiconductor devices are formed; a mask layer formed on the substrate to have an opening which corresponds to a device isolating region; a trench region formed by etching the substrate using the mask layer as an etching mask; and an insulating layer filled in the trench region so that an upper edge or upper corner of the trench region is not exposed.

The upper edge of the trench region may be rounded-off. The mask layer may be formed so that the opening is expanded outwardly to cover the upper edge of the trench region completely.

According to a second aspect of the present invention, a method includes the steps of providing a substrate on which a plurality of semiconductor devices are formed; performing a first etching treatment to the substrate with a first etching gas comprising CF₄ to form a base trench having a rounded-off upper edge or tapered upper edge; performing a second etching treatment to the substrate to form a trench region at the base trench so that the trench region has a rounded-off upper edge; and forming an insulating layer on the substrate to fill up the trench region therewith.

The first etching gas may include HBr. Preferably, the ratio of flow rate of HBr and CF₄ is between 1:2 and 1:5.

The first etching gas may further include CH₂F₂. Preferably, the ratio of flow rate of CF₄ and CH₂F₂ is between 2:1 and 3:1. Further, the upper edge of the trench region may be rounded off before forming the insulating layer.

According to a third aspect of the present invention, a method includes the steps of: providing a substrate on which a plurality of semiconductor devices are formed; providing a mask layer on the substrate to have an opening corresponding to a device isolating region; performing a first etching treatment to the substrate using the mask layer as an etching mask to form a trench region on the substrate; enlarging the opening of the mask layer so that an upper edge of the trench region are fully exposed; and providing an insulating layer on the substrate so that the insulating layer extends outwardly from the trench region to cover the upper edge of the trench region completely.

Preferably, the opening of the mask layer is enlarged in the range of 300 Å to 500 Å in a horizontal direction on the substrate. The opening of the mask layer may be enlarged by an isotropic etching process.

Further, a thermal oxidation treatment may be carried out to the substrate after enlarging the opening of the mask layer so that the upper edge of the trench region is rounded off.

A second etching treatment may be carried out to the substrate after the opening of the mask layer is enlarged so that the upper edge of the trench region is tapered. In this case, preferably, a thermal oxidation treatment is carried out to the substrate after the second etching treatment so that the tapered upper edge of the trench region is rounded off.

The upper edge of the trench region may be rounded off before enlarging the opening of the mask layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a part of a conventional semiconductor apparatus.

FIGS. 2A to 2G are cross-sectional views showing the fabrication steps according to a first preferred embodiment of the present invention.

FIGS. 3A and 3B are cross-sectional view used for showing shapes of a semiconductor substrate treated in the different ways.

FIGS. 4A to 4H are cross-sectional views showing the fabrication steps according to a second preferred embodiment of the present invention.

FIGS. 5A to 5G are cross-sectional views showing the fabrication steps according to a third preferred embodiment of the present invention.

FIGS. 6A to 6H are cross-sectional views showing the fabrication steps according to a fourth preferred embodiment of the present invention.

FIGS. 7A to 7H are cross-sectional views showing the fabrication steps according to a fifth preferred embodiment of the present invention.

FIGS. 8A to 8J are cross-sectional views showing the fabrication steps according to a sixth preferred embodiment of the present invention.

DETAILED DISCLOSURE OF THE INVENTION

In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which forma part hereof, and in which is shown by way of illustration specific preferred embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present inventions. The following detailed description is, therefore, not to be taken in a limiting sense, and scope of the present inventions is defined only by the appended claims.

For better understanding of the present invention, a conventional technology is first described in conjunction with FIG. 1.

According to a conventional method, an SiN layer (not shown) is formed on a semiconductor substrate 102; then trench regions 103 are formed by etching using the SiN layer as a mask. After that, an oxide layer 104 is provided in the trench regions 103 by a CVD process; and then, an upper portion of the oxide layer 104 is removed by a CMP process.

According to such a conventional method, sharp corners 103 a of the trench regions 103 may be exposed when the SiN layer is removed. As a result, characteristics of semiconductor devices may be changed, and electric field is concentrated at the exposed corners 103 a of the trench regions 103.

First Preferred Embodiment

FIGS. 2A to 2G are cross-sectional views showing the fabrication steps according to a first preferred embodiment of the present invention.

First, referring to FIG. 2A, a PAD oxide layer 2 is formed on the semiconductor substrate 1 by a CVD process to have a thickness of about 200 Å. Next, an SiN layer 3 is formed on the PAD oxide layer 2 by a CVD process to have a thickness of about 500 Å to 5000 Å.

Subsequently, a resist layer is formed on the SiN layer 3 and is patterned to have openings 4 a corresponding to device isolating regions, as shown in FIG. 2B.

Now referring to FIG. 2C, the PAD oxide layer 2 and SiN layer 3 are plasma-etched with an etching gas including HBr and CF₄ using the resist pattern 4 as an etching mask to form trench bases 1 b, at which trench regions 5 are to be formed. The etching process used in this embodiment is ICP (Inductively Coupled Plasma) type of etching. In this embodiment first and second etching processes are carried out to form the trench regions 5 in the substrate 1. A first etching is carried out under the following conditions:

(1) Gas Flow Rate: HBr/CF₄=20/80 sccm

(2) Electric Power to Upper Electrode/Lower Electrode: 700 W/150 W

(3) Pressure: 15 mTorr

(4) Etching Time: 10 sec

According to the first etching process, the trench bases 1 b with rounded-off edges 1 a are formed in the substrate 1. CF₄ is included in the etching gas, so that no sharp corners or upper edges are formed but the rounded-off edges 1 a are formed. It can be considered that a reaction product including fluorine is formed on inner side surfaces of the SiN layer 3, and therefore, the etching speed is lowered around inside surface of the trench bases 1 b. In contrast, according to a conventional method using an etching gas not including CF₄, trench regions are formed to have sharp corners of edges.

Shapes of the rounded-off edges 1 b are different depending on the ratio of HBr and CF4 in the etching gas. FIG. 3A shows a trench region 5 formed by an etching process using an etching gas including HBr and CF₄ at the ration of 4:1. FIG. 3B shows a trench region 5 formed by an etching process using an etching gas including HBr and CF₄ at the ration of 1:4. The following is preferable condition for the first etching:

(1) Ratio of HBr:CF₄=1:2 to 1:5

(2) Electric Power to Upper Electrode: 500 W or higher

(3) Pressure: 10 to 30 mTorr

(4) Etching Time: 10 to 20 sec

After first etching process, a second etching process is carried out with an etching gas excluding CF₄ to form the trench regions 5 in the substrate 1, as shown in FIG. 2D. The second etching process is carried out under the following condition:

(1) Gas Flow Rate: HBr/O₂=90/5 sccm

(2) Electric Power to Upper Electrode/Lower Electrode: 600 W/66 W

(3) Pressure: 5 mTorr

(4) Etching Time: 85 sec

“The first and second etching processes are carried out continuously using the same etching chamber. The trench regions 5 are formed to have a depth of about 2500 Å to 5000 Å. Each of the trench regions 5 is shaped to have a narrower bottom and wider top. Shaping the trench regions in that way makes an oxide 6 layer go into the bottom of the trench region 5 easily. The first and second etching processes can be carried out only using the SiN layer 3 as an etching mask after removing the resist pattern 4. However, in view of simplicity of fabrication steps and deformation of the SiN layer 3, both of the resist layer 4 and SiN layer 3 are preferably used as an etching mask.”

Next, as shown in FIG. 2E, an embedded oxide layer 6 is formed over the entire surface of the substrate 1 by a CVD process using high-density plasma. The trench regions 5 are filled up with the oxide layer 6.

Subsequently, the oxide layer 6 is polished by a CMP process to the surface level of the SiN layer 3, as shown in FIG. 2F. After that, the SiN layer 3 and PAD oxide layer 2 are removed to form device isolating regions, as shown in FIG. 2G.

As described above, according to the first preferred embodiment of the present invention, the trench regions 5 are formed to have rounded-shaped upper edges, so that concentration of electric field at upper edges of the trench regions can be prevented and characteristics of semiconductor devices becomes stable and reliable.

Second Preferred Embodiment

FIGS. 4A to 4H are cross-sectional views showing the fabrication steps according to a second preferred embodiment of the present invention. In this embodiment, the same and corresponding components to those in the first preferred embodiment will be represented by the same reference numerals.

First, referring to FIG. 4A, a PAD oxide layer 2 is formed on the semiconductor substrate 1 by a CVD process to have a thickness of about 200 Å. Next, an SiN layer 3 is formed on the PAD oxide layer 2 by a CVD process to have a thickness of about 500 Å to 5000 Å.

Subsequently, a resist layer is formed on the SiN layer 3 and is patterned to have openings 4 a corresponding to device isolating regions, as shown in FIG. 4B.

Now referring to FIG. 4C, the PAD oxide layer 2 and SiN layer 3 are plasma-etched with an etching gas including HBr, CF₄ and CH₂F₂ using the resist pattern 4 as an etching mask to form trench bases 1 b, at which trench regions 5 are to be formed. The etching process used in this embodiment is ICP (Inductively Coupled Plasma) type of etching. In this embodiment first and second etching processes are carried out to form the trench regions 5 in the substrate 1. A first etching is carried out under the following conditions:

(1) Gas Flow Rate: HBr/CF₄/CH₂F₂=20/60/20 sccm

(2) Electric Power to Upper Electrode: 400 W

(3) Pressure: 15 mTorr

(4) Etching Time: 10 sec

According to the first etching process, the trench bases 1 b with tapered edges 1 c are formed in the substrate 1. The tapered corners 1 c are formed when the etching gas includes CF₄ and CH₂F₂ at a rate of 2:1 to 3:1.

After first etching process, a second etching process is carried out with an etching gas excluding CF₄ to form the trench regions 5 in the substrate 1, as shown in FIG. 4D. The second etching process is carried out under the following condition:

(1) Gas Flow Rate: HBr/O₂=90/5 sccm

(2) Electric Power to Upper Electrode/Lower Electrode: 600 W/66 W

(3) Pressure: 5 mTorr

(4) Etching Time: 85 sec

The first and second etching processes are carried out continuously using the same etching chamber.

After the second etching process, a thermal oxidation process is carried out to the substrate 1 at a temperature of 1050° C. to 1100° C. to round off upper corners 5 a of the trench regions 5, as shown in FIG. 4E.

The trench regions 5 are formed to have a depth of about 2500 Å to 5000 Å. Each of the trench regions 5 is shaped to have a wider bottom and narrower top. Shaping the trench regions in that way makes an oxide 6 layer go into the bottom of the trench region 5 easily. The first and second etching processes can be carried out only using the SiN layer 3 as an etching mask after removing the resist pattern 4. However, in view of simplicity of fabrication steps and deformation of the SiN layer 3, both of the resist layer 4 and SiN layer 3 are preferably used as an etching mask.

Next, as shown in FIG. 4F, an embedded oxide layer 6 is formed over the entire surface of the substrate 1 by a CVD process using high-density plasma. The trench regions 5 are filled up with the oxide layer 6.

Subsequently, the oxide layer 6 is polished by a CMP process to the surface level of the SiN layer 3, as shown in FIG. 4G. After that, the SiN layer 3 and PAD oxide layer 2 are removed to form device isolating regions, as shown in FIG. 4H.

As described above, according to the second preferred embodiment of the present invention, CH₂F₂ gas is used in the first etching process, so that the tapered corners 1 b are reliably formed with the trench regions 5. Further, the tapered corners 1 b are rounded-off, and therefore, concentration of electric field at upper edges of the trench regions 5 can be prevented and characteristics of semiconductor devices becomes stable and reliable.

Third Preferred Embodiment

FIGS. 5A to 5G are cross-sectional views showing the fabrication steps according to a third preferred embodiment of the present invention. In this embodiment, the same and corresponding components to those in the above-described embodiments will be represented by the same reference numerals.

First, referring to FIG. 5A, a PAD oxide layer 2 is formed on the semiconductor substrate 1 by a CVD process to have a thickness of about 200 Å. Next, an SiN layer 3 is formed on the PAD oxide layer 2 by a CVD process to have a thickness of about 500 Å to 5000 Å.

Subsequently, a resist layer is formed on the SiN layer 3 and is patterned to have openings 4 a corresponding to device isolating regions, as shown in FIG. 5B.

Now referring to FIG. 5C, the PAD oxide layer 2 and SiN layer 3 are plasma-etched by a conventional method with an etching gas of HBr using the resist pattern 4 as an etching mask to form trench regions 5. The first etching process is carried out under the following condition:

(1) Gas Flow Rate: HBr/O₂=90/5 sccm

(2) Electric Power to Upper Electrode/Lower Electrode: 600 W/66 W

(3) Pressure: 5 mTorr

(4) Etching Time: 85 sec

After that, an isotropic etching is carried out to the SiN layer 3 by a chemical reaction type of etching using H₃PO₄ or downstream type of plasma etching so that the openings 4 a are enlarged or expanded outwardly from the trench regions 5, as shown in FIG. 5D. Preferably, the SiN layer 3 is chemically etched using H₃PO₄ to reduce etching damage to the silicon substrate 1. The chemical etching may be carried out using H₃PO₄ at 160° C. for about 10 minutes.

The trench regions 5 are formed to have a depth of about 2500 Å to 5000 Å. Each of the trench regions 5 is shaped to have a wider bottom and narrower top. Shaping the trench regions in that way makes an oxide 6 layer go into the bottom of the trench region 5 easily.

Next, as shown in FIG. 5E, the embedded oxide layer 6 is formed over the entire surface of the substrate 1 by a CVD process using high-density plasma. The trench regions 5 are filled up with the oxide layer 6.

Subsequently, the oxide layer 6 is polished by a CMP process to the surface level of the SiN layer 3, as shown in FIG. 5F. After that, the SiN layer 3 and PAD oxide layer 2 are removed to form device isolating regions, as shown in FIG. 5G.

The openings defined by the SiN layer 3 and PAD oxide layer 2 are enlarged, so that the oxide layer 6 completely cover all the corners (upper edges) of the trench regions 5, as shown in FIG. 5G. In other words, the oxide layer 6 is formed to extend outwardly from the trench regions 5 onto active regions surrounding the trench regions 5 on the semiconductor substrate 1. The portions extended on the active regions prevent that upper edges (corners) of the trench regions 5 are exposed when the SiN layer 3 and PAD oxide layer 2 are removed.

Preferably, the openings defined by the SiN layer 3 are enlarged or expanded by a length of L, 300 Å to 500 Å, and the oxide layer 6 is formed to have a thickness of 300 Å to 500 Å in order to protect the upper edges of the trench regions 5. If the oxide layer 6 has a thickness less than 300 Å, the oxide layer 6 might be undesirably removed at the upper edges of the trench regions 5 when the SiN layer 3 and PAD oxide layer 2 are removed; and as a result, the upper edges of the trench regions 5 would be exposed. For that reason, the openings defined by the SiN layer 3 are enlarged or expanded by a length of L, more than 300 Å, and the oxide layer 6 is formed to have a thickness larger than 300 Å.

If the openings defined by the SiN layer 3 are enlarged by a length or width more than 500 Å, the width of the SiN layer 3 would be too short. As a result, the oxide layer 6 may be polished too much.

As described above, according to the third preferred embodiment of the present invention, the oxide layer 6 is formed so as to cover the upper edges of the trench regions 5 completely; and therefore, reliable device characteristics can be obtained.

Fourth Preferred Embodiment

FIGS. 6A to 6H are cross-sectional views showing the fabrication steps according to a fourth preferred embodiment of the present invention. In this embodiment, the same and corresponding components to those in the above-described embodiments will be represented by the same reference numerals.

First, referring to FIG. 6A, a PAD oxide layer 2 is formed on the semiconductor substrate 1 by a CVD process to have a thickness of about 200 Å. Next, an SiN layer 3 is formed on the PAD oxide layer 2 by a CVD process to have a thickness of about 500 Å to 5000 Å.

Subsequently, a resist layer is formed on the SiN layer 3 and is patterned to have openings 4 a corresponding to device isolating regions, as shown in FIG. 6B.

Now referring to FIG. 6C, the PAD oxide layer 2 and SiN layer 3 are plasma-etched by a conventional method with an etching gas of HBr using the resist pattern 4 as an etching mask to form trench regions 5. The first etching process is carried out under the following condition:

(1) Gas Flow Rate: HBr/O₂=90/5 sccm

(2) Electric Power to Upper Electrode/Lower Electrode: 600 W/66 W

(3) Pressure: 5 mTorr

(4) Etching Time: 85 sec

After that, an isotropic etching is carried out to the SiN layer 3 by a chemical reaction type of etching using H₃PO₄ or downstream type of plasma etching so that the openings 4 a are enlarged or expanded outwardly from the trench regions 5, as shown in FIG. 6D. Preferably, the SiN layer 3 is chemically etched using H₃PO₄ to reduce etching damage to the silicon substrate 1. The chemical etching may be carried out using H₃PO₄ at 160° C. for about 10 minutes.

The trench regions 5 are formed to have a depth of about 2500 Å to 5000 Å. Each of the trench regions 5 is shaped to have a wider bottom and narrower top. Shaping the trench regions in that way makes an oxide 6 layer go into the bottom of the trench region 5 easily.

Next, a thermal oxidation treatment is carried out to the semiconductor substrate 1 to round off upper edges 5 a of the trench regions 5, as shown in FIG. 6E. Such a thermal oxidation treatment is a dry type at a temperature of 1050° C. to 1100° C.

It is known that oxidation characteristics of such a thermal oxidation process not only depend on oxidation condition but also on physical stress. More precisely, oxidation speed is faster at a flat portion, where physical stress is dispersed, and is slower at a shape portion, to which physical stress is easily concentrated, such as the upper edges of the trench region. According to a conventional trench region, upper edges are covered with an SiN layer, so that the upper edges are hardly oxidized. According to this embodiment, the semiconductor substrate 1 is oxidized after the SiN layer 3 is withdrawn or retracted backwardly, so that the upper edges 5 a of the trench regions 5 are rounded off easily without affection of stress of the SiN layer 3.

When such a thermal oxidation treatment is carried out, a thermal oxidation layer or coat is formed on the surface of the trench regions 5, and at the same time, the rounded-off portions 6 a are formed.

Next, as shown in FIG. 6F, the embedded oxide layer 6 is formed over the entire surface of the substrate 1 by a CVD process using high-density plasma. The trench regions 5 are filled up with the oxide layer 6.

Subsequently, the oxide layer 6 is polished by a CMP process to the surface level of the SiN layer 3, as shown in FIG. 6G. After that, the SiN layer 3 and PAD oxide layer 2 are removed to form device isolating regions, as shown in FIG. 6H.

The openings defined by the SiN layer 3 and PAD oxide layer 2 are enlarged, so that the oxide layer 6 completely cover all the corners (upper edges) of the trench regions 5, as shown in FIG. 6H. In other words, the oxide layer 6 is formed to extend outwardly from the trench regions 5 onto active regions surrounding the trench regions 5 on the semiconductor substrate 1. The portions extended on the active regions prevent that upper edges (corners) of the trench regions 5 are exposed when the SiN layer 3 and PAD oxide layer 2 are removed.

As described above, according to the fourth preferred embodiment of the present invention, the oxide layer 6 is formed so as to cover the upper edges 5 a of the trench regions 5 completely; and therefore, reliable device characteristics can be obtained. In addition, the upper edges 5 a are rounded-off, so that concentration of electrical field at the upper edges of the trench regions can be prevented.

Fifth Preferred Embodiment

FIGS. 7A to 7H are cross-sectional views showing the fabrication steps according to a fifth preferred embodiment of the present invention. In this embodiment, the same and corresponding components to those in the above-described embodiments will be represented by the same reference numerals.

First, referring to FIG. 7A, a PAD oxide layer 2 is formed on the semiconductor substrate 1 by a CVD process to have a thickness of about 200 Å. Next, an SiN layer 3 is formed on the PAD oxide layer 2 by a CVD process to have a thickness of about 200 Å to 5000 Å.

Subsequently, a resist layer is formed on the SiN layer 3 and is patterned to have openings 4 a corresponding to device isolating regions, as shown in FIG. 7B.

Now referring to FIG. 7C, the PAD oxide layer 2 and SiN layer 3 are plasma-etched by a conventional method with an etching gas of HBr using the resist pattern 4 as an etching mask to form trench regions 5. The first etching process is carried out under the following condition:

(1) Gas Flow Rate: HBr/O₂=90/5 sccm

(2) Electric Power to Upper Electrode/Lower Electrode: 600 W/66 W

(3) Pressure: 5 mTorr

(4) Etching Time: 85 sec

After that, an isotropic etching is carried out to the SiN layer 3 by a chemical reaction type of etching using H₃PO₄ or downstream type of plasma etching so that the openings 4 a are enlarged or expanded outwardly from the trench regions 5, as shown in FIG. 7D. Preferably, the SiN layer 3 is chemically etched using H₃PO₄ to reduce etching damage to the silicon substrate 1. The chemical etching may be carried out using H₃PO₄ at 160° C. for about 10 minutes.

The trench regions 5 are formed to have a depth of about 2500 Å to 5000 Å. Each of the trench regions 5 is shaped to have a wider bottom and narrower top. Shaping the trench regions in that way makes an oxide 6 layer go into the bottom of the trench region 5 easily.

Next, another trench etching process is carried out to the semiconductor substrate 1. Upper corners of the trench regions 5 are exposed, so that the corners are tapered 5 b, as shown in FIG. 7E. That is because, etching speed is faster at the corners due to concentration of electric field.

Next, a thermal oxidation treatment is carried out to the semiconductor substrate 1 to round off the tapered edges 5 b of the trench regions 5, as shown in FIG. 7F, so that rounded-off edges 5 a are formed. Such a thermal oxidation treatment is a dry type at a temperature of 1050° C. to 1100° C. The rounded-off edges 5 a are little gentler or looser as compared to those in the fourth preferred embodiment.

Next, as shown in FIG. 7F, the embedded oxide layer 6 is formed over the entire surface of the substrate 1 by a CVD process using high-density plasma. The trench regions 5 are filled up with the oxide layer 6.

Subsequently, the oxide layer 6 is polished by a CMP process to the surface level of the SiN layer 3, as shown in FIG. 7G. After that, the SiN layer 3 and PAD oxide layer 2 are removed to form device isolating regions, as shown in FIG. 7H.

The openings defined by the SiN layer 3 and PAD oxide layer 2 are enlarged, so that the oxide layer 6 completely cover all the corners (upper edges) of the trench regions 5, as shown in FIG. 7H. In other words, the oxide layer 6 is formed to extend outwardly from the trench regions 5 onto active regions surrounding the trench regions 5 on the semiconductor substrate 1. The portions extended on the active regions prevent that upper edges (corners) of the trench regions 5 are exposed when the SiN layer 3 and PAD oxide layer 2 are removed.

As described above, according to the fifth preferred embodiment of the present invention, the oxide layer 6 is formed so as to cover the upper edges 5 a of the trench regions 5 completely; and therefore, reliable device characteristics can be obtained. In addition, the upper edges 5 a are rounded-off, so that concentration of electrical field at the upper edges of the trench regions 5 can be prevented.

Sixth Preferred Embodiment

FIGS. 8A to 8J are cross-sectional views showing the fabrication steps according to a sixth preferred embodiment of the present invention. In this embodiment, the same and corresponding components to those in the above-described embodiments will be represented by the same reference numerals.

First, referring to FIG. 8A, a PAD oxide layer 2 is formed on the semiconductor substrate 1 by a CVD process to have a thickness of about 200 Å. Next, an SiN layer 3 is formed on the PAD oxide layer 2 by a CVD process to have a thickness of about 200 Å to 5000 Å.

Subsequently, a resist layer is formed on the SiN layer 3 and is patterned to have openings 4 a corresponding to device isolating regions, as shown in FIG. 8B.

Now referring to FIG. 8C, the PAD oxide layer 2 and SiN layer 3 are plasma-etched by a conventional method with an etching gas of HBr using the resist pattern 4 as an etching mask to form trench regions 5. The first etching process is carried out under the following condition:

(1) Gas Flow Rate: HBr/O₂=90/5 sccm

(2) Electric Power to Upper Electrode/Lower Electrode: 600 W/66 W

(3) Pressure: 5 mTorr

(4) Etching Time: 85 sec

After that, an isotropic etching is carried out to the SiN layer 3 by a chemical reaction type of etching using H₃PO₄ or downstream type of plasma etching so that the openings 4 a are enlarged or expanded outwardly from the trench regions 5, as shown in FIG. 8D. Preferably, the SiN layer 3 is chemically etched using H₃PO₄ to reduce etching damage to the silicon substrate 1. The chemical etching may be carried out using H₃PO₄ at 160° C. for about 10 minutes. The resist pattern 4 is then removed.

The trench regions 5 are formed to have a depth of about 2500 Å to 5000 Å. Each of the trench regions 5 is shaped to have a wider bottom and narrower top. Shaping the trench regions in that way makes an oxide 6 layer go into the bottom of the trench region 5 easily.

Next, a first thermal oxidation treatment is carried out to the semiconductor substrate 1 to round off upper edges 5 a of the trench regions 5, as shown in FIG. 8E. Such a thermal oxidation treatment is a dry type at a temperature of 1050° C. to 1100° C.

After that, an isotropic etching is carried out to the SiN layer 3 by a chemical reaction type of etching using H₃PO₄ so that the openings 4 a are enlarged or expanded outwardly from the trench regions 5, as shown in FIG. 8F. When the first thermal oxidation treatment, a thin oxide layer is formed on the surface of the silicon substrate 1, so that the inner surface of the trench regions 5 are not exposed to H₃PO₄ directly.

Next, as shown in FIG. 8G, the embedded oxide layer 6 is formed over the entire surface of the substrate 1 by a CVD process using high-density plasma. The trench regions 5 are filled up with the oxide layer 6. Preferably, the oxide layer 6 is of porous structure or a layer containing water or moisture.

After that, a second thermal oxidation process is carried out to the silicon substrate 1 to further round off the upper edges 5 a of the trench regions 5, as shown in FIG. 8H. Preferably, the second thermal oxidation is of wet manner, in which oxidation speed is faster, rather than dry manner, in which oxidation speed is slower. Although, the upper edges 5 a of the trench regions are covered with a CVD oxide layer, round-off effect to the upper edges 5 a can forward, because the physical stress of the CVD oxide layer is lower than that of the SiN layer 3. The oxide layer is preferably of porous structure to obtain sufficient round-off effect. The oxide layer 6 becomes to have a higher density in the second thermal oxidation process.

Subsequently, the oxide layer 6 is polished by a CMP process to the surface level of the SiN layer 3, as shown in FIG. 8I. After that, the SiN layer 3 and PAD oxide layer 2 are removed to form device isolating regions, as shown in FIG. 8J.

The openings defined by the SiN layer 3 and PAD oxide layer 2 are enlarged, so that the oxide layer 6 completely cover all the corners (upper edges) of the trench regions 5, as shown in FIG. 8J. In other words, the oxide layer 6 is formed to extend outwardly from the trench regions 5 onto active regions surrounding the trench regions 5 on the semiconductor substrate 1. The portions extended on the active regions prevent that upper edges (corners) of the trench regions 5 are exposed when the SiN layer 3 and PAD oxide layer 2 are removed.

As described above, according to the sixth preferred embodiment of the present invention, the oxide layer 6 is formed so as to cover the upper edges 5 a of the trench regions 5 completely; and therefore, reliable device characteristics can be obtained. In addition, the upper edges 5 a are rounded-off, so that concentration of electrical field at the upper edges of the trench regions can be prevented. Further, the silicon substrate 1 is not exposed to H₃PO₄, so that stable and reliable device characteristics can be obtained. 

What is claimed is:
 1. A method for fabricating a semiconductor apparatus, comprising: providing a substrate having a principal surface; performing a first etching treatment on the substrate with a first etching gas comprising CF₄ and HBr to form a base trench having a rounded-off upper edge or tapered upper edge, wherein a flow rate of CF₄ is larger than a flow rate of HBr; performing a second etching treatment on the substrate to form a trench region at the base trench so that the trench region has a rounded-off upper edge; and forming an insulating layer on the substrate to fill up the trench region therewith.
 2. A method according to claim 1, wherein a ratio of flow rate of HBr and CF₄ is between 1:2 and 1:5.
 3. A method according to claim 1, wherein the first etching gas further comprises CH₂F₂.
 4. A method according to claim 3, wherein a ratio of flow rate of CF₄ and CH₂F₂ is between 2:1 and 3:1.
 5. A method according to claim 3, further comprising: rounding-off the upper edge of the trench region before said forming an insulating layer.
 6. A method according to claim 5, wherein said rounding-off the upper edge is carried out by a thermal oxidation treatment of the substrate.
 7. A method according to claim 6, wherein the thermal oxidation treatment is carried out at a temperature of 1050° C. to 1100° C.
 8. A method for fabricating a semiconductor apparatus, comprising: providing a substrate on which a plurality of semiconductor devices are formed; performing a first etching treatment on the substrate with a first etching gas comprising HBr and CF₄ to form a base trench having a rounded-off upper edge; performing a second etching treatment on the substrate using a second etching gas excluding CF₄ to form a trench region at the base trench so that the trench region has a rounded-off upper edge; and forming an oxide layer on the substrate to fill up the trench region therewith, wherein a ratio of flow rate of HBr and CF₄ in the first etching gas is between 1:2 and 1:5.
 9. A method for fabricating a semiconductor apparatus, comprising: providing a substrate on which a plurality of semiconductor devices are formed; performing a first etching treatment on the substrate with a first etching gas comprising HBr, CF₄ and CH₂F₂ to form a base trench having a tapered upper edge; performing a second etching treatment on the substrate using a second etching gas excluding CF₄ and CH₂F₂ to form a trench region at the base trench; performing a thermal oxidation treatment on the substrate so that the trench region has a rounded-off upper edge; and forming an oxide layer on the substrate to fill up the trench region therewith, wherein a ratio of flow rate of CF₄ and CH₂F₂ in the first etching gas is between 2:1 and 3:1.
 10. A method according to claim 9, wherein the thermal oxidation treatment is carried out at a temperature of 1050° C. to 1100° C. 